Use of wood as a fuel for residential heating has increased dramatically, resulting in a concomitant increase in air pollution. Airtight wood stoves, the type generally employed for residential heating, regulate heat output by throttling air supplied to a primary combustion chamber, to produce fuel-rich conditions that commonly cause unburned combustibles to be exhausted from the stove when the stove is operated at low heat rates. Most woodstoves used in the United States are generally operated at low burn rates because the stoves are located in a room being heated. If the stoves were operated at high burn rates, persons located in the room would become excessively warm. Most United States residential users of woodstoves prefer large stoves, which they operate at low burn rates, because such stoves need not be constantly filled with wood fuel. Burning the fuel at low burn rates, however, has the disadvantage of relatively low efficiency and high pollution because significant unburnt combustibles are exhausted from the stove.
Many design alternatives have been evaluated to provide more complete woodstove combustion. One of the most promising woodstove designs involves two chambers in the stove. A primary combustion chamber contains burning wood and includes an inlet, i.e. damper, for limited air entry. Usually, the damper is not fully open to provide a medium or low burn rate to sustain a smoldering fire that volatizes wood fuel. The resulting combustion gases pass into a secondary chamber, where combustion is theoretically completed with the aid of additional air introduced into the secondary combustion chamber from outside the stove. Theoretically, the advantage of this arrangement is that the fuel volatilization rate is decoupled from the combustion process so that complete combustion is achieved in the secondary chamber when low burn rates occur in the primary chamber.
It has been found that sustaining combustion in the woodstove secondary combustion chamber is difficult, at best, for low or medium burn rates in the primary chamber. The combustibles from the primary chamber and the air introduced into the secondary chamber must be well mixed. The composition of the mixture must be within flammability limits and the temperature of the mixture in the secondary chamber must exceed ignition temperature of the mixture therein. In a woodstove, the volatilization rate and the chemical composition of the combustibles change throughout a burn cycle. The flow rate of air introduced into the secondary chamber, which is usually naturally aspirated, changes with the flow rate of gases from the primary chamber into the secondary chamber, as does turbulence which causes mixing. The secondary chamber temperature also changes during a burn cycle.
The recommended procedure for operating certain woodstoves having secondary combustion chambers is to establish, for about one-half hour, a high burn rate in the primary combustion chamber with a damper to the secondary chamber closed. The temperature on the stove exterior may reach 800.degree.-900.degree. F. under these conditions. This operation causes combustion to occur in the secondary chamber when the damper to the secondary chamber is opened, and helps to remove tar and creosote from the walls of the chimney. However, a great deal of fuel is required to achieve this high burn rate and the room where the stove is located usually attains an excessively high temperature. Hence, this high burn rate operation is inefficient, although it is conducive to combustion in the secondary chamber immediately after the interchamber damper is opened. However, because of the high room temperature attained during the high burn rate operation, air flowing into the primary chamber is often severely throttled by closing a damper into the primary chamber when the interchamber damper is opened. This leads to a low or, at most, medium burn rate in the primary chamber, frequently, causing combustion to be quenched in the secondary chamber, so that desiderata of the secondary chamber are not achieved for a prolonged period.
Prior art woodstove design modifications to control the widely varying conditions in the primary and secondary chambers have attempted to promote sustained secondary chamber combustion. In attempts to sustain high temperature and combustion in the secondary chamber, the prior art has suggested: (1) preheating the supply of air fed into the secondary chamber, (2) regulating the composition of gases supplied to the secondary chamber, and (3) an external ignitor in the secondary chamber; see Allen et al. "Control of Emissions from Residential Woodburning by Combustion Modification" U.S. Environmental Protection Agency Report EPA-600/7-81-091 (1981). The effectiveness in reducing emissions of the first of these three concepts has been tested in the laboratory; see the aforementioned Allen et al. article, as well as Allen et al. "Control of Woodstove Emissions Using Improved Secondary Combustion" U.S. Environmental Protection Agency Report EPA-600/7-84-061 (1984); and Knight et al. "Efficiency and Emission Performance of Residential Wood Heaters with Advanced Designs," Proceedings of the APCA 76th Annual Meeting Atlanta, Georgia, 1983. In general, with these prior art techniques, emissions were reduced when the stove was operated at high burn rates. However, little or no emission reduction was observed at the low burn rates commonly used during "steady state" operation in United States residences.
Other techniques for sustaining combustion in the secondary combustion chamber have involved the use of a natural gas powered flame and electrical ignitors; see Spolek et al., "Secondary Combustion in a Dual-Chamber Woodstove," ASHRAE Transactions Vol. 91, Part 1, pages 1138-1146, 1988. Laboratory measurements of woodstove emissions using natural gas powered flames have demonstrated a substantial decrease during limited testing. However, experimentation with natural gas powered flames was suspended because of practical problems associated with supplying an external natural gas source to a woodstove. In experiments we conducted with electrical ignitors, wherein the ignitors were located on the secondary combustion chamber outside wall, it was found that the electrical ignitor did not result in complete combustion of products in the secondary chamber.
An object of the invention is to provide a new and improved dual chamber woodstove having secondary chamber combustion control and method of operating same.
It is another object of the present invention to provide a new and improved dual chamber woodstove having a secondary combustion chamber wherein the stove is efficiently operated and emissions, including particulates, are substantially reduced, even though fuel is being burned in a primary combustion chamber of the stove at a medium or low burn rate.
An additional object of the invention is to provide a new and improved dual chamber woodstove having automatic control of combustion in a secondary combustion chamber of the stove.